A PM motor (Permanent Magnet Synchronous Motor) as a rotary machine has features of being compact and highly efficient, and has recently been widely used for industrial equipment and the like. However, because of its structure, the PM motor contains a harmonic component in its induced voltage and thus has a torque ripple in its produced torque. This may result in causing a problem of vibration, noise, mechanical resonance, or the like, so that a reduction technique of the ripple becomes necessary.
As the reduction technique, there is known that which reduces a torque ripple by overlapping a compensation signal for reducing the torque ripple on a current command (see, for example, Patent Document 1).